In telephone installations the subscriber sets are connected to the exchange by so-called subscriber lines, which are terminated in the exchange at an adaption circuit, or subscriber line interface circuit (SLIC). Such a circuit is used, inter alia, to supply DC current to the associated subscriber apparatus.
A fixed DC voltage may be utilized for this current supply in the prior art. In such a case, the current flowing through the subscriber line will be a function of the internal resistance of the subscriber apparatus or station, and of course the line length. The internal resistance in the apparatus can be kept within narrow limits, but the line length and line resistance may vary considerably. If a fixed DC voltage is used, this must be given a value such that sufficient current can be fed out on long lines as well.
In modern line circuits, the supply impedances are often simulated with the aid of feed-back amplifiers. The physical supply resistances are low-ohmic and the power generation in the resistance is low. In order to reduce the power generation in the output step of the amplifier as well, it is known to utilize controllable DC/DC converters, e.g. so-called choppers, instead of batteries in the voltage supply.
Two principles are described in the literature for the impedance simulation, c.f. Proceedings Of the IEEE, Vol. 68, No. 8, August 1980, pages 991-1009, for example. According to one principle, the line current is sensed to form an instant value in a control system controlling the line voltage. This method gives simple conditions for operating point setting of the participating amplifiers and control of DC/DC converters. However, the method has the considerable disadvantage that the amplifier which has floatingly to sense the line current out on the line will be difficult to achieve with sufficient precision.
According to the other method, the line voltage is sensed and the control system controls the line current. This method gives more simple conditions for realizing the sensing amplifier, but on the other hand it gives difficulties in setting the operating points of the amplifiers and control of DC/DC converters.
In electronic subscriber line circuits according to the above, the amplifiers require a supply voltage for their function which exceeds by a given amount the transversal line voltage, i.e. the voltage between the outputs of the amplifiers. If this condition is not met, there is the risk of the signal voltage being clipped. This relationship defines the maximum length of a line which can be connected to the line circuit without risk of transmission degeneration. Consideration must also be taken here of the fact that the battery voltage can vary considerably, depending on the state of charge, load etc.